1. Field of the Invention
The present invention relates to a preparation of a Pb-substituted hydroxyapatite catalyst for oxidative coupling of methane in which methane is dimerized at a high yield when a dimerizing reaction is performed for producing ethane and ethylene by partially oxidizing methane which is a major constituent of natural gas with oxygen.
2. Description of the Conventional Art
Methane is the most stable hydrocarbon which is a major constituent of natural gas and much attention has been drawn to methane as a chemical feed stock and an energy source to serve as a substitute for petroleum. Therefore, active studies toward the production of C.sub.2 compound by dimerizing methane have been performed. C.sub.2 compounds such as ethane and ethylene are very useful as raw material for producing polyethylene and various copolymers of ethylene and propylene or other monomers. Moreover, the dimerization reaction increases the utility of methane to work as a substitute energy source for petroleum, since the C.sub.2 compounds are more stable during transportation than methane. When methane is completely combusted, CO.sub.x is formed, and C.sub.2 compounds which are intermediate products have higher reactivities with oxygen than methane. Therefore, it is very difficult to selectively obtain C.sub.2 compounds under an oxidizing condition.
Many studies have been made for the development of an oxidation catalyst having a high selectivity for the dimerization of methane including J. Catal. 73, 9 by G. E. Keller and M. M. Bhasin (1982), and the catalysts representing a comparatively high activity thereof are lithium/magnesium oxide (Li/MgO), samarium oxide (Sm.sub.2 O.sub.3) and lead oxide/magnesium oxide (PbO/MgO). However, in the case of catalysts well known so far, the yield of C.sub.2 compounds is at most about 25%, and in addition, the catalyst is deactivated over the reaction time. Therefore, a commercial process for producing the C.sub.2 compounds has not yet been developed.
A supported lead oxide catalyst studied by W. Hinsen, W. Bytyn, M. Baerns (Proc. 8th ICC,3,581 (1984)) is one of the catalysts which have been studied most widely (J.A.S.P. Carreiro and M. Baerns, Reac. Kinet. Catal. Lett., 35,349 (1987)). The activity and selectivity of the catalyst vary depending on the ratio of methane/oxygen, temperature, additives, the support and the Pb-loading. The selectivity for ethylene is high when the lead oxide is supported on a basic oxide or prepared as a complex oxide with a basic oxide. Particularly, the PbO/MgO catalyst exhibits a C.sub.2 selectivity as high as 80% on the condition of a lower ratio of oxygen/methane J. P. Bartek, J. M. Hupp, J. F. Brazdil and R. K. Grasselli, Catal. Today, 3,117 (1988)!. However, when the catalytic reaction is carried out at a high temperature in order to achieve a higher conversion, the selectivity is sharply lowered and the lead oxide is evaporated, J. A. Ross, A. G. Bakker, H. Bosch, J. G. van Ommen and J. R. H. Ross, Catal. Today, 1,133 (1987)!.
As described previously, for the stabilization of a lead catalyst, studies using a non-volatile salt or a complex oxide have been continued.
Generally, a hydroxyapatite catalyst is a compound which is obtained by the reaction of calcium nitrate tetrahydrate Ca(NO.sub.3).sub.2 4H.sub.2 O!, ammonium phosphate (NH.sub.4).sub.3 PO.sub.4 ! and an aqueous ammonia (NH.sub.4 OH), and a Pb-substituted hydroxyapatite catalyst is the hydroxyapatite catalyst in which lead (Pb) is substituted for calcium (Ca). The Pb-substituted hydroxyapatite catalyst produced by using an ion-exchange method was reported to be active for C.sub.2 dimerization reaction. However, the highest yield of the reaction. was no more than 13%, which is not sufficient for commercialization. A Pb-substituted hydroxyapatite catalyst was produced by a cation exchange to be employed in a C.sub.2 dimerizing action, but the maximum yield was below 20%, which is also not sufficient K. Yamashita, H. Owada, H. Nakagawa, T. Umegaki, and T. Kanaawa, J. Am. Ceram. Soc., 69,590(1986)!.